1. Field of the Invention
The invention relates to an antenna apparatus using an antenna unit with length shorter than ¼ the transmission or reception wavelength, and a mobile communication device using the same.
2. Description of the Related Art
Design goals for personal mobile communication devices or wireless terminal equipment focus on light weight, thinness, compact profile and good communication quality. Mobile phones, for example, feature small streamlined models with multiple functions and applications, and low cost.
Wireless communication equipment requires more than one antenna such that multiple communication systems can be integrated and applied. Hence, it becomes a design issue and a critical problem to arrange the antennas in the wireless communication equipment and determine dimensions of antennas. In mobile phones, GSM systems of 850/900/1800/1900 MHz, WCDMA or CDMA2000 system, Bluetooth system and wireless LAN of 2400 or 5200/5800 MHz may be integrated into a single mobile phone, requiring more than 3 antennas. Fortunately, GSM, WCDMA, CDMA2000, Bluetooth and wireless LAN systems all operate in the frequency band above 1 GHz, such that necessary antennas can be configured in the mobile phone with great effort by designing the antennas based on ¼ wavelength antenna.
However, it is difficult to integrate additional wireless communication systems, such as HF, VHF or UHF system operating in the frequency band below 1 GHz, into a single wireless communication equipment like a mobile phone, because the dimension of the antenna for HF, VHF or UHF system is too large to be configured into the mobile phone when designing ¼ wavelength antenna. Manufacturers either configure HF, VHF or UHF antenna outside the housing of the mobile, or enlarge dimensions of the mobile phone to enclose large HF, VHF or UHF antenna, therefore degrading elegant profile of the mobile phone and increasing production costs. FM radio systems operate in the frequency range of about 88˜108 MHz within VHF band. External earphones are required to serve as antennas for reception thereof, when providing FM radio systems in mobile phones. However, it is inconvenient for users to prepare earphones to listen FM radio.
FIG. 1 shows equivalent circuit diagram of an antenna apparatus with length shorter than ¼ the operating wavelength, disclosed by U.S. Pat. No. 6,970,140. The antenna apparatus 100 comprises an antenna unit 11, a resistor 12 and a matching circuit 13, connected in series in this sequence. Generally, real part of input impedance (i.e., input resistance) of the antenna unit 11 is very small and less than real part of input impedance of a circuit unit 14. Hence, impedance matching (using only the matching circuit 13) can not increase the input resistance of the antenna unit 11 to equal that of the circuit unit 14. To solve this problem, U.S. Pat. No. 6,970,140 further discloses coupling the resistor 12 between the antenna unit 11 and the matching circuit 13. However, the resistor 12 consumes considerable energy transmitted between the antenna unit 11 and the circuit 14, degrading communication performance.
Accordingly, it is desirable to have an antenna apparatus having an antenna unit with length shorter than ¼ the operating wavelength of the antenna apparatus. In addition, the antenna unit on a first plane can be arranged to set a first distance or a first angle with respect to a ground plane of the antenna apparatus, thereby improving impedance matching between the antenna unit and a communication module coupled thereto, and increasing radiation efficiency thereof. Consequently, the antenna apparatus can be applied to mobile communication devices without affecting appearance or cost.